Welcome to an exciting journey into the development of the most Activision-fucked game ever for at least the next month, going over a lot of remains in the menu files... which contain very interesting stuff.
We'll start off with a simple menu, 'auconfirm', which was likely used during development.
Even though this wasn't the menu name that triggered my interest in checking them out, the menu below is...
... 'Log Into IW.net'. This menu tells quite a story, as it appears originally the game wasn't even meant to use Steam as online platform -- but 'IW.net', which would most likely be a full custom platform, possibly based on DemonWare technology as in World at War. This doesn't necessarily mean that the PC branch was already scrapped by then, but evidence that follows might. More evidence (an unused 'IW.net storage server' protocol) also shows this platform was at least meant to provide remote storage services, similar to Steam Cloud, and a popular missing feature in CoD4.
Also of note is the name 'IW.net', which can also be seen in a console message 'Could not resolve iw.net matchmaking server hostname', while it's obvious the released product is named 'IWNet'. It might be a purchase of the domain name 'iw.net' was planned during the time of it being a larger platform, but either that didn't turn out, or other time constraints called for the usage of Steam, and the cutting down of the IWNet service.
Some more MW2-styled IW.net dialog windows follow...
... and, oddly, some CoD4-styled IW.net ('Modern Warfare 2 Account') windows as well:
These show that at least the 'online platform' portion of the 'IW.net' system was planned quite early into development, most likely even in 2008, and it appears to have been scrapped somewhere in the final stages of development.
Also, a nice-looking and fully-functional (outside of, obviously, code backing missing) player profile system was implemented as well:
Some more interesting stuff can be found in a menu labeled menu_systemlink, which is one of the few console-named menus to appear in the PC version...
... with a platform-specific string ('System Link' on Xbox) named 'LAN Party'. This shows that proper LAN support was planned to be implemented into MW2 on PC, but piracy concerns at Activision caused this feature to be dropped quite easily. As a further note, the 'Create a Class' and 'Callsign & Killstreaks' buttons properly function, the 'Barracks' function tries to open the non-existent menu 'menu_offline_barracks', and the Join Game button, well...
... you'd expect it to open the menu shown below ...
... but it does not. Instead, it opens this menu that made me go 'UFGRGFERUFGH484UHRUEHFURGH4RJ' on IRC...
... a fucked version of the CoD4 server browser. Was it that, back then, an advanced browsing menu was planned for the LAN Party option, or was it something completely else?
To close off the story, a few other interesting menus are shown below.
The CoD4 'start server' UI.
The same UI, with MW2 map images in the CoD4 list.
Ooh, a remain from CoD4.
A prototype for MW2 CaC.
Helmets? It appears that perks changing appearance wasn't something invented by Treyarch...
A very early killstreak reward UI. Note the 'Double UAV'...
UNBALANCED! AC130 AT 9 KILLS!
Hm, which of these three options should we remove? The minigunner is fun... the nuke pwns... we'll remove the Grenadier as a nuke is absolutely not unbalanced!
... hm, shall I pick UAV or UAV as my first killstreak reward?
Mods menu from CoD4.
... at least it doesn't say 'Lanuch without Mods' anymore...
Source: http://alteriw.net/viewtopic.php?f=85&t=21065
